The invention relates to a continuous process for the manufacture of nitrobenzene. This process comprises the nitration of benzene with a nitrating acid that contains at least 3.0 wt. % of nitric acid and at least 67.0 wt. % of sulfuric acid, in a reaction space in which the start temperature of the reaction is above 100.0° C. but below 102.0° C. In addition, the process requires that the benzene and the nitrating acid are dispersed in one another several times.
The present invention relates to a continuous process for the manufacture of nitrobenzene that essentially corresponds to the concept of the adiabatic nitration of benzene with a mixture of sulfuric and nitric acids (nitrating acid). Such a process was disclosed in U.S. Pat. No. 2,256,999 and current embodiments of this adiabatic nitration are now described in, for example, EP 0 436 443 B1, EP 0 771 783 B1 and U.S. Pat. No. 6,562,247. The processes with an adiabatic mode of reaction are particularly distinguished by the fact that heating or cooling energy fluxes do not flow over the outer surface of the reaction volume by means of technical measures.
A common feature of the adiabatic processes described above is that the starting materials, i.e. benzene and nitric acid, are reacted in a large excess of sulfuric acid. The sulfuric acid then takes up the evolved heat of reaction and the water formed in the reaction.
To carry out the reaction, nitric acid and sulfuric acid are generally mixed or dispersed to form so-called nitrating acid, and benzene is metered into this nitrating acid and reacts with the nitric acid to form substantially water and nitrobenzene. Benzene is used in at least the stoichiometric required amount, based on the amount of nitric acid, but preferably in a 2 to 10% excess, relative to the amount of benzene required by stoichiometry.
The reaction space in which benzene and nitric acid are reacted can consist of an arrangement of stirred tanks, a loop reactor or one or more tubular reactors connected in series or parallel. The tubular reactors can be of cylindrical or conical design. It is advantageous to operate the tubular reactors without back-mixing, so the magnitude of the flow rate inside the tubular reactor is chosen so that plug-flow behaviour is achieved over the whole of the reactor.
The dimensions of the reaction space are generally such that a reaction mixture substantially free of nitric acid is obtained after the reactants have flowed through it. To describe the course of the reaction in the reaction space, it is useful conceptually to subdivide the latter into a start zone, in which the emulsion of benzene and nitrating acid is first produced by dispersion and in which the reaction starts and the reactants are optionally redispersed several times, and an adjoining end zone, in which the reaction proceeds to almost complete conversion of the nitric acid. In this process, the start zone physically begins with the first dispersion of benzene and nitrating acid. It is again possible, if desired, conceptually to subdivide this zone into several zones. The physical and chemical processes in a nitration apparatus are described in detail in EP 0 708 076 B1.
Downstream of the end zone of the nitration reactor, the reaction mixture is generally fed into a phase separation apparatus, where two phases are formed; the first phase being crude nitrobenzene and the second phase waste acid which consists substantially of water and sulfuric acid. At the same time, gases—substantially nitrogen oxides, water vapor and benzene vapor—are evolved from the liquid phase in the phase separation apparatus. These gases are generally fed into an exhaust gas system.
The waste acid obtained in the phase separation apparatus is conventionally fed into a flash evaporator, where water is evaporated as the waste acid is expanded into the vacuum, thereby cooling and concentrating the waste acid. The adiabatic mode of nitrating benzene with nitrating acid has the advantage that the heat of the exothermic reaction is utilized to heat the waste acid to the point where the concentration and temperature in the flash evaporator can simultaneously be adjusted to those of the sulfuric acid before the admixing of nitric acid and benzene.
By way of impurities, the nitrobenzene obtained in the phase separation apparatus (so-called crude nitrobenzene) still contains sulfuric acid, water and benzene, as well as nitrophenols and dinitrobenzene, which are separated off by suitable work-up processes, e.g. washing and distillation steps. One possible embodiment of this work-up is described in, for example, EP 1 816 117 A1.
Features of the adiabatic nitration of aromatic hydrocarbons are firstly that the temperature of the reaction mixture increases in proportion with the progress of the reaction, i.e. with the nitric acid conversion. Secondly, this creates a difference between the temperature of the mixed educts before the onset of the reaction (hereafter described as the start temperature) and the temperature of the reaction mixture after at least 99% nitric acid conversion (hereafter described as the reaction end temperature). Those skilled in the art are aware that, in general, the variable described here as the start temperature can advantageously be measured immediately after the benzene has been metered into the nitrating acid, i.e. before the first dispersion, and the variable described here as the reaction end temperature can be measured in the inlet of the phase separation apparatus. The difference between the start temperature and reaction end temperature depends on the type of hydrocarbon being nitrated and on the volumetric proportions in which the aromatic hydrocarbon and the nitrating acid have been used. A high volumetric ratio of aromatic hydrocarbon to nitrating acid (also called the phase ratio) gives a large temperature difference and has the advantage that a large amount of aromatic hydrocarbon is converted per unit time.
In industrial practice, such as in the case of the nitration of benzene, the reaction end temperature is limited by safety criteria to approx. 135 to 145° C., with the decisive role being determined by factors such as the thermal decomposition of the products and the vapor pressure of the product mixture, and hence the temperature and quantity of the exhaust gas. In view of this prescribed upper limit, it has previously seemed obvious to choose the lowest possible start temperature because this increases the difference relative to the reaction end temperature applicable on the industrial scale, and it becomes possible to achieve high space-time yields.
The most important criterion for describing the quality of an adiabatic process for the nitration of aromatic hydrocarbons is the end product's content of undesirable reaction by-products formed by multiple nitration or oxidation of the aromatic hydrocarbon or the nitroaromatic. In the case of the nitration of benzene, discussion always centers on the content of dinitrobenzene and nitrophenols, especially trinitrophenol (picric acid), which is to be classified as particularly explosive.
In order to obtain nitrobenzene with particularly high selectivities, the nature of the nitrating acid to be used has been determined in detail (see, for example, EP 0 373 966 B1, EP 0 436 443 B1 and EP 0 771 783 B1), and numerous suggestions have been put forward as to how the first mixing and the repeat mixing (redispersion) of benzene with the nitrating acid can be carried out (see EP 0 373 966 B1, EP 0 489 211 B1, EP 0771 783 B1, EP 0 779 270 B1, EP 1 291 078 A2 and U.S. Pat. No. 6,562,247). It has also been pointed out that the content of by-products is determined by the magnitude of the end temperature (cf. EP 0 436 443 B1, column 15, lines 22-25), that a high initial conversion is advantageous for a high selectivity, and that this is achieved when the intermixing effected at the beginning of the reaction is optimal (EP 0 771 783 B1, paragraph 0014). In EP 0 771 783 B1, the required intermixing is achieved by means of a rotating propulsive jet. The use of dispersing elements, as described in, for example, EP 0 489 211 B1 and EP 0 436 443 B1, is regarded in EP 0 771 783 B1 as unsuitable (see paragraphs 0012 to 0015).
All the cited processes are capable of producing nitrobenzene by the nitration of benzene with nitrating acid, an adiabatic mode of operation being used in order to heat the sulfuric acid with the heat of reaction and thereby be able to use the heat of reaction to concentrate the waste acid. All the processes described are capable of producing a crude nitrobenzene having a low content of by-products, i.e. containing only between 100 and 300 ppm of dinitrobenzene and between 1500 and 2500 ppm of nitrophenols, with it being possible for picric acid to make up 10 to 50% of the nitrophenols.
Furthermore, irrespective of this, the purity of the crude nitrobenzene is of decisive importance for industrial production. However, against the background of the increasing demand for nitroaromatics, especially for the manufacture of aromatic amines and aromatic isocyanates, there is another objective, namely to provide the possibility of manufacturing large amounts of these compounds in reaction equipment of maximum compactness.
The extensive state of the art scarcely goes into the dimensions of the proposed mixing equipment and reactors. Only EP 0 779 270 B1, however, gives concrete numerical data for a laboratory reactor.
One parameter for describing the ratio between the quantity which can be produced and the size of the reaction equipment is the space-time yield. This is calculated as the quotient of the producible quantity of target compound per unit time and the volume of the reaction equipment. In the present case of the nitration of benzene, the space-time yield is usefully calculated as the quotient of the nitrobenzene production in metric tons per hour and the volume of the reaction space, the latter being defined as the space which begins with the first dispersion of benzene and nitrating acid and within which the reaction proceeds to at least 99% completion. The residence time of the reaction mixture (consisting of aromatic compound and nitrating acid) within the reaction space is the reaction time.space-time yield [t/m3h]=quantity produced [t/h]/reaction space [m3]
A high space-time yield is advantageous for the industrial application of a process because it makes it possible to construct compact reaction equipment distinguished by a low volume of investment relative to capacity.
The aim of the present invention is thus to provide a process which on the one hand affords a high space-time yield, and on the other hand continues to assure the required product quality.
It has surprisingly been found that the space-time yield can only be significantly increased, while maintaining product quality, when all the reaction-accelerating factors, i.e. the composition of the nitrating acid, the start temperature and the quality of the intermixing, are coordinated in such a way that, at the beginning of the reaction in a start zone making up at most 13 vol. % of the reaction space, at least 60% of the nitric acid used reacts with benzene to form nitrobenzene.
To increase the space-time yield, the state of the art makes provision for increasing the phase ratio and lowering the start temperature in order to obtain a greater difference relative to the reaction end temperature applicable on the industrial scale. It has been surprisingly found, however, that these measures, while increasing the space-time yield for nitrobenzene, are not capable of also maintaining the required product quality and the required process control.
To achieve space-time yields of more than 5 t/m3h while maintaining product quality, it is necessary to resort to two essential measures compared with a process according to the state of the art:    1. Raising the start temperature. This inevitably implies a higher reaction end temperature; however, when applying the combination of measures described here, this makes only a negligible contribution to the effect described in EP 0 436 443 B1, namely an increase in the content of by-products.    2. The reaction mixture must be redispersed several times at short intervals immediately after benzene has been metered in. If the dispersion frequency exceeds a preferred value of ⅕ s−1, i.e. if there is on average a period (corresponding to a residence time) preferably of more than 5 seconds between two (re)dispersions, increasing the space-time yield to more than 5 t/m3h leads to an impairment of product quality.
Surprisingly, it has been found that, by applying both the above measures, neither raising the start temperature alone (Comparative Example 1) nor increasing the dispersion frequency alone (Comparative Example 2) makes it possible to guarantee the manufacture of nitrobenzene with a space-time yield of 5.1 t/m3h. In both cases the values of important process parameters, such as exhaust gas temperature and nitric acid conversion, are above or below the limiting values. It is only when both measures are combined in such a way as to satisfy the criterion according to the invention, i.e. namely at least 60% nitric acid conversion in a start zone making up at most 13 vol. % of the reaction space, that nitrobenzene can be produced with a space-time yield of more than 5 t/m3h. (See Example 1 according to the invention). If the composition of the nitrating acid is additionally optimized, the space-time yield can also be increased to well above 5 t/m3h. (See Example 2 according to the invention).